1. Field of the Invention
The present invention relates to an ink jet printing apparatus that forms an image by ejecting ink from a print head onto a print medium, and more particularly to an ink jet printing apparatus with a post-processing unit which, after a printing operation, forms a protective layer over a printed medium by performing a lamination on the surface of the printed medium using a thermal head. This ink jet printing apparatus can function in a printer, copying machine or facsimile machine, or as an output device for a combination machine such as a computer or word processor or as an output device for a workstation.
2. Description of the Related Art
A variety of kinds of printing apparatus has been used for an input device which, according to print information, outputs various images (including characters and symbols) to different kinds of print media (printing materials). The printing apparatus may be categorized according to a printing method employed by a printing means used, such as an ink jet printing apparatus, a wire dot printing apparatus, a thermal printing apparatus, a sublimation transfer printing apparatus, an electrophotographic printing apparatus and a silver salt photographic printing apparatus.
Of these, the ink jet printing apparatus ejects ink droplets (including droplets of printing performance improving liquid) from nozzles of a print head. Because of its ability to perform printing without bringing the print head into contact with a print medium, this ink jet printing apparatus is quiet during the printing operation and can print a high resolution image at high speed on a variety of print media, from plain paper to rough print media, without requiring any special processing. It also has advantages of an ease with which it can print color images using multiple color inks, a low manufacturing cost and a low running cost.
Particularly in the case of a printing means (print head) of a so-called Bubble Jet (trademark) type, in which a bubble is generated in ink by thermal energy produced by an electrothermal transducer to eject an ink droplet by a pressure of the bubble as it grows, a high-density liquid path arrangement (nozzle arrangement) can be realized by performing a semiconductor device manufacturing process, including etching, deposition and sputtering, to form the electrothermal transducers, electrodes, liquid path walls and ceilings on a substrate. Therefore, the print head of this printing method can be constructed compactly.
The ink jet printing apparatus can be classified largely into a serial type and a full line type. The serial type ink jet printing apparatus prints an image (including characters and symbols) on a print medium set at a predetermined printing position by reciprocally moving the printing means (print head) along with a carriage in a main scan direction. After the print head has printed one line of data, the print medium is fed a predetermined distance in a subscan direction. By repeating the printing action and the print medium feeding action, an image is printed on the print medium in a desired range.
In the full line type ink jet printing apparatus, the printing means is secured at a fixed position and performs printing by feeding the print medium in the subscan direction to form an image on the entire area of the print medium.
The present invention can be applied to either of these types. In the following explanation, a serial type ink jet printing apparatus, which is most popular as a general purpose ink jet printing apparatus, will be taken as an example.
FIG. 24 is a perspective view schematically showing a construction of a printing unit 20 of a serial type ink jet printing apparatus in wide use.
In FIG. 24, designated by reference numeral 1 is a printing means having a plurality of print heads that eject ink droplets onto a print medium for forming an image. Here, four kinds of print heads 1Y, 1C, 1M, 1Bk are provided that eject four colors of ink, yellow, cyan, magenta and black. Denoted by reference numeral 2 is an ink supply unit 19 that supply inks to the associated print heads. There are four ink tanks storing the four colors of ink, yellow, cyan, magenta and black.
A transport roller 23 is driven by a paper feed motor (not shown) to move a print medium 23a in the form of continuous paper or a cut sheet. The transport roller 23 rotates with high precision to determine the distance that the print medium 23a is moved.
Print media used for ink jet printing are made from a material capable of absorbing a liquid ink well and having a characteristic such that it can easily absorb water and other substances even after an image has been formed. Suppose a water-absorbing print medium already formed with an image is to be printed further. Printing on such a print medium with an ink containing a water-soluble ink or alcohol solvent may cause the already formed image to bleed, which is undesirable. Further, if an inert gas coming out of a resin of transparent file, such as vinyl chloride and polypropylene, or tobacco smoke is present around printed media, the media may absorb contaminating substances resulting in the fading of the printed image.
As described above, a print medium formed with an image by ink jet printing has drawbacks of low water resistance, low weather resistance and, therefore, low permanence of the printed image. Other drawbacks reside in that an irregularity appears on an outer surface of a printing medium when a material having a good ink absorbing characteristic is applied to the printing medium in such a manner constituting a porous structure (more than the structure of an ink coloring material) in order for a better ink absorbing characteristic, and that an irregularity of a surface of a base material appears on the outer surface of the printing medium when using the base material having a good ink absorbing characteristic, respectively, resulting in degradation of a texture of the printing medium, e.g., the printing medium after printing may lack a glossy surface. When, on the other hand, the print medium used is made of a glossy film as a base material, a relatively glossy print can be obtained, but another problem arises in that because applied ink droplets must be absorbed only by a coating at a top layer, the ink absorbing performance is degraded. To deal with this problem, it has conventionally been proposed that after an image is printed, post-processing be performed, which involves laminating the surface of the printed medium with a transparent or translucent film or sheet-like member, or applying an oil or wax agent to the medium surface.
However, in the post-processing that applies a post-processing liquid such as an oil or wax agent to the printed medium after printing, there is a difference in a post-processing liquid absorbing capacity between an area that has already absorbed ink and an area that has not yet absorbed it, resulting in causing non-uniformity of the post-processing liquid between the areas. To cope with this problem it has been proposed that the printed medium be dried by a drying means before performing the post-processing so that the post-processing liquid can be applied uniformly over an entire area including those locations where the ink has been absorbed. This method, however, requires a drying process to fix the applied post-processing liquid on the print medium, making the apparatus large in size.
On the other hand, a printing apparatus that performs a lamination on the surface of the printed medium as by a heat transfer method can be constructed relatively compact and is recognized for its ability to enhance weatherability and water resistance.
Examples of apparatus that perform laminations on the surfaces of printed media include Japanese Patent Application Laid-open Nos. 62-161583 (1987) and 2001-232782. Here, let us turn to FIG. 25 to explain about a printing apparatus that has a post-processing unit for performing lamination.
A printing apparatus shown in FIG. 25 has an ink jet printing unit 20 similar in construction to that shown in FIG. 24. This printing apparatus, like the one shown in FIG. 24, performs the printing operation by main-scanning the print head 1 in the direction of arrows Sa, Sb, while at the same time feeding the print medium 23a intermittently in the direction of arrow Sy.
In coordination with the scanning of the print head 1, the print medium 23a is fed a predetermined distance with a high precision by a pair of transport rollers 23. The print head 1 ejects ink from its nozzles by using, for example, thermal energy.
In FIG. 25, the print medium 23a is schematically shown to be continuous, from a pre-printing feeding unit up to a post-processing unit 70. In reality, however, the print medium has a maximum recording length so set that, when the printing is finished, the maximum recording length lies a predetermined distance in front of the post-processing unit in the feeding direction.
Until the printing operation is completed, the print medium is fed a predetermined distance as the printing action of the print head proceeds. Then, during the post-processing operation by the post-processing unit 70, the print medium is transported continuously at a constant speed. In a process of switching between the two different transport actions, the above-described predetermined distance plays a role as a buffer area. After having been printed with an image, the print medium 23a is led by paired rollers into the post-processing unit 70.
The post-processing unit 70 has a full line type thermal head 300 employing a known heat transfer method, a platen roller 210 opposing the thermal head, a supply roller 81A having a transfer film F wound on it, and a takeup roller 81B for winding up the transfer film F fed from the supply roller 81A. The transfer film F extending from the supply roller 81A to the takeup roller 81B engages the thermal head 300 and is transported with an even, constant tension.
In the post-processing unit 70 of the above construction, when the print medium 23a is supplied into the post-processing unit, the thermal head 300 applies heat to the print medium 23a over an image-printed width or a width of the print medium. As a result, transparent resin or wax or both are transferred from the transfer film F onto the printing surface of the print medium 23a to form a transparent protective layer. At this time, a base material of the transfer film carrying the protective layer (the base material is made of, for example, polyethylene tereprithalate or PET) is wound up on the takeup roller 81B for disposal after use.
In this printing apparatus, in which the transparent protective layer is heated and transferred onto the print medium in the post-processing unit, there is a problem that an optimum amount of heat applied for film transfer varies depending on the amount of water absorbed in the surface of the print medium. That is, in areas that have absorbed a large volume of water in the top layer of the print medium, the heat capacity of water is large. This means that when these areas are heated, the water evaporates to dissipate heat, preventing the temperature at these areas from rising sufficiently. Conversely, in areas with a small volume of water, the heat capacity of water is small, so that upon heating the temperature rises too much. Therefore, the film transferability greatly varies according to the amount of water contained in the print medium, making it impossible to secure a uniform and stable transferability. Such a tendency becomes more conspicuous as an average volume of ink increases, as when using dark and light inks, and also as the printing speed increases.
The amount of water absorbed in the print medium is greatly affected by the amount of ink ejected onto the print medium during the ink jet printing process, by an environment surrounding the print medium (temperature and humidity), and by a time it takes from when the ink has landed on the print medium until a lamination starts (affecting the amount of water in the print medium that evaporates). Hence, with an ink jet printing apparatus with a conventional lamination unit, it is extremely difficult to form a uniform protective layer on the print medium stably.
In a printing apparatus that prints image data by using a print head of a thermal transfer printing methods, a construction for controlling a drive pulse according to image data, a drive history of heat transfer printing input pulses or old print data is described in, for example, Japanese Patent Nos. 2570715, 2879784 and 3088520. This conventional printing apparatus, however, simply uses a heat transfer print head for printing image data, rather than using it for laminating print media. That is, the heat transfer print head used in the conventional printing apparatus is not intended to make the print medium lamination uniform.
A method of controlling, according to print data, a condition of fixing a printed image formed by ink jet printing is proposed in Japanese Patent No. 2761671. A construction described in this patent, however, is not intended for lamination, but for uniformly drying a printed medium after ink on the medium has temporarily been dried.